There is now compelling evidence to suggest that epidermal growth factor (EGF) is involved in the control of spermatogenesis. Much of this evidence comes from functional and biochemical studies which enumerated different Sertoli and Leydig cell activities regulated by EGF. In addition, experimental paradigms in which mice have been made EGF deficient, either by submandibular gland ablation or chemical treatment, result in either azoospermia or oligozoospermia. Nevertheless, the vital step(s) regulated by EGF is still not clear; in particular, does EGF serve to control the proliferative phase of spermatogonial mitoses? Moreover, although the submandibular gland of mice is known to be the principal tissue source of EGF, more recent evidence suggests that the testis itself can synthesize the EGF precursor, although it is not clear that this precursor is then processed to the mature, active peptide. Therefore, two specific aims are describe to study in detail the role of EGF in spermatogenesis. First, the hypothesis that EGF regulates spermatogonial division will be investigated. Two experimental paradigms were designed to test directly whether 1) EGF depletion leads to a deleterious effect on spermatogonial division and whether 2) replacement of EGF either maintains or restores spermatogonial mitoses. Second, having ascertained that the testis is capable of de novo synthesis of the EGF precursor, functional activity of this precursor molecule will be investigated: 1) The EGF precursor of isolated pachytene spermatocytes will be characterized biochemically to determine if it is processed to the biologically active EGF polypeptide. Alternatively, 2) the ability of the intact, unprocessed precursor to activate Sertoli cell EGF receptor by a juxtacrine interaction will be investigated. We suggest that this latter mechanism may be a fundamental new means to understand how spermatogenesis in regulated. This knowledge may prove useful in designing novel strategies to reinstate spermatogenesis, to limit spermatogenesis specifically in the absence of systemic effects, or to block the aberrant division of germ cells and somatic cells of the testis during the cancer state.